The postnatal development of adrenergic receptor mechanisms is being studied in piglets. Work in progress includes analysis of cardiovascular responses to adrenergic receptor blocking agents and to electrical stimulation of efferent sympathetic nerves to heart and kidneys, and will be extended to include responses to stimulation of lumbar efferents to the hindlimb. Adult control experiments are carried out on mature miniature swine. The responses under study include heart rate, arterial and ventricular pressures and regional blood flows (cephalic, renal, femoral), and utilize strain gauge pressure transducer and electromagnetic flowmeter recordings. The dose-response relationship to increasing frequency and duration of electrical stimulation of the nerves is being evaluated, and the effective receptor-blocking dose of a given pharmacologic agent is being determined in different age groups of animals. The existence of age-dependency for any difference in magnitude of cardiovascular response to adrenergic stimulation or receptor blockade is being established statistically in a sufficiently large population of animals in each age group. This research is expected to provide knowledge on the role of peripheral mechanisms in the autonomic regulation of cardiovascular function in the neonate.